super high pfc

[c.boy]

Working in a high rise block of flats bout just under half way up!! changed the c/u now my ze is 0.01 prob parralell paths,however my pfc is 19.9 ka!!! is this supply problem or because its high rise flats!! 3 lead method used live to neutral(black and green lead together) and live to neutral with green lead on earth bar for pfsc both readings bout the same any ideas would be much appreciated!!

thanks

 

[beaver74]

Working in a high rise block of flats bout just under half way up!! changed the c/u now my ze is 0.01 prob parralell paths,however my pfc is 19.9 ka!!! is this supply problem or because its high rise flats!! 3 lead method used live to neutral(black and green lead together) and live to neutral with green lead on earth bar for pfsc both readings bout the same any ideas would be much appreciated!!

thanks

I would say it will be a problem as you will now have to get breakers with a breaking capacity of 19KA min

 

[NoSparks]

Unfortunately I agree. Some of those high Rise had the transformer in the basement. So assuming you are maybe 15 meters above the transformer and that the feed is 25mm concentric, your calculated Ze would be somewhere between 0.01 and 0.02.

Never actually come across this situation myself. Are 20,000 Amp breakers available? Or is there any other way of increasing the resistance?

 

[Des 56]

80 and 100 amp 1361 fuses can be rated at 33 KA
This is the protection
The 6Ka mcb does not need to match the PFC

Is this reading you are obtaining accurate ?
The consumer units are generally only rated at 16.5 KA by the manufacturers

 

[ian.settle1]

80 and 100 amp 1361 fuses can be rated at 33 KA
This is the protection
The 6Ka mcb does not need to match the PFC

Is this reading you are obtaining accurate ?
The consumer units are generally only rated at 16.5 KA by the manufacturers

As to the 1361 fuses rated at 33ka or any fuses for that manner this is no problem as when the fuse blows you have to replace the fuse.

The problem is when using MCBs as protection as their rating is either 6 or 10Ka so whilst they will trip and possibly reset they will need to be replaced if you read the manufactures guide they are damaged and it is not guarenteed they work work correctly after this.

 

[c.boy]

hi
yeah re tested today with another tester and same again!! done a ground floor flat today my ze was 0.32 and my pfc was 3.5 ka doing the sums should be about 718a. how can a rectify this as my boss is so strict on the test certs and he will tell me to fix this but how can i alter the supply and earthing into each flat??? am stumped and no happy!!! just glad these are empty propertys at the mo!!all of my d/bs are disconnected at the main cut out till i get to the bottom of this!!!

 

[Des 56]

Change your board or breakers by all means ,the installation is adequately protected by the BS1361 II rated at 33kA

 

[NoSparks]

Sorry but I don't understand why a 6kA breaker, which would be the first protective device to try opening, would be protected by a BS1361 service fuse. Suppose there were a short circuit on the lighting (ie as I had just a week ago) then the MCB would attempt to open in maybe even less than 1 second while the service fuse would probably not break at all. The damage to the MCB would be irreprablel.

 

[Notsosmart]

Sorry but I don't understand why a 6kA breaker, which would be the first protective device to try opening, would be protected by a BS1361 service fuse. Suppose there were a short circuit on the lighting (ie as I had just a week ago) then the MCB would attempt to open in maybe even less than 1 second while the service fuse would probably not break at all. The damage to the MCB would be irreprablel.

Yes but it is allowable for a 6kA mcb to be backed up by an appropriate HBC fuse. Look up 'Back-up protection' in BS7671 (section 536) in the case of this post Preferably10kA mcb's should be selected.

In an ideal world the manufacturers would produce all mcb's capable of withstanding up to 24kA ie zero impedance short circuit.

 

[NoSparks]

I've been researching the concept of backup. What I have found is that it is permissable to have two protection devices in series where the first is capable of the full shortcircuit load, and the second to have a lower short circuit capacity and a lower current limit. eg a 33kA device protecting a 6kA device.

However, the let through energy of the first device must be less than that which would damage the second device.

In manufacturers literature it is said that this can be achieved by an MCCB reacting in 10mS (ie half a wave) and in this cace a 32kA MCCB can protect a 6kA MCB, but it is stressed that the devices must be matched and tested under laboritory conditions.

I can well see an 80A BS1361 rated at 33kA protecting a HRC fuse, and indeed there are references to "fuses backing up fuses" with no specific examples.

What concerns me is that I cannot find the let through energy of a BS1361 fuse, or the maximum energy that a 6kA MCB can take without damage. At 19kA the BS1361 will cut out in 100mS but this is far greater than the the 10mS recomended by the manufacturers and therefore looks unsuitable.

Would anybody care to give figures that would support such an arrangement?